Cyclic ring systems

Organic ring systems are named by replacement nomenclature. Three- to ten-membered mono-cyclic ring systems containing uncharged boron atoms may be named by the specialist nomenclature for heterocyclic systems. Organic derivatives are named as outlined for substitutive nomenclature. The complexity of boron nomenclature precludes additional details the text by Rigaudy and Klesney should be consulted. 

Intramolecular 1,3-cycloadditions of nitrile oxides (INOC) provide a useful tool for the construction of fused cyclic ring systems. The stereochemical outcome of this reaction is presumed to be a consequence of reaction through the transition state that minimizes allylic 1,3 strain (Scheme 8.19).103... 

Reggelin et al. reported the application of methylated, enantiomerically pure acyclic and cyclic 2-alkenyl sulfoximines 203 for the synthesis of highly substituted aza(poly)cyclic ring systems 204 under complete stereocontrol <06JA4023 06S2224>. 

When the diene used is cydopentadiene, endo-cyclic ring systems result such as are also produced by vegetable cells in camphor and other terpenes. 

Ester functions present in molecules tend to be considered labile although steric effects etc. may be utilized to produce drugs without inherent chemical or metabolic problems due to ester lability. For instance a series of antimuscarinic compounds which had selectivity for the M3 receptor (Figure 7.18) were stabilized by the incorporation of a hydroxy ethyl side chain or a cyclic ring system at positions surrounding the ester function. Presumably the proximity of these groups to the ester function (carbonyl) prevents close approach of the attacking nucleophile, in this case probably a serine hydroxyl. 

Similar compounds were chosen by selecting untested compounds with common cyclic ring systems using the SBI descriptors (43). 

Fig. 8. Comparison of the different ring sizes present in the NCI with the Pharmacia CAC pharmaceutical library. (A) Histogram of the distribution of ring system size present in the NCI dataset. Note the approx 2000 compounds with no ring system and the large number of very large cyclic ring systems (larger than 30 and as large as 95, NCI compound 683343). (B) By comparison the CAC library contains very few compounds with no cyclic system and the largest cyclic system is 28.

Further research on this subject was recently reported, in relation to the use of dienynes as substrates for intramolecular cycloaddition. While thermal intramolecular [4+2] cycloadditions of enynes with alkenes only took place at high temperatures, the gold(I) catalyzed transformations provided bi- or tri-cyclic ring systems under mild conditions [152]. 

Synthesis of TVi- and Poly-cyclic Ring Systems without Ring Junction Heteroatoms 657... 

For a homoantiaromatic system, surface delocalization in the cyclopropyl ring is parallel to the bridging bond, thus forming a Mobius antiaromatic electron ensemble delocalized along the periphery of the bi(poly)cyclic ring system. 

This chapter will focus on preparative applications of iron catalysts in the synthesis of cyclic ring systems by means of intra- and intermolecular cycloadditions, Alder-ene reactions and ring expansion reactions. Previous reviews concerning iron-catalyzed chemistry have either focused their attention on different aspects involving iron-containing compounds as active catalysts [1] or have concentrated on certain reactions [2] and on the synthesis of specific substance classes [3]. A more recent general review concerning all aspects of modern applications in iron-catalyzed reactions has been summarized by Bolm et al. [4]. 

The cationic center is planar unless it is incorporated into a (poly)cyclic ring system and extant pyramidalization becomes inevitable. 

The two carbene units can be embedded in a (macro)cyclic ring system known as a cyclophane. A standard procedure for the synthesis of such a system starts with a,a -dibromoxylene and potassium imidazolide [368]. Cychsation can be achieved by reacting the bis-imidazole compound with a second equivalent of a,a -dibromoxylene (see Figure 3.116). The cyclic bis-imidazolium cyclophane can then be reacted with paUadium(II) acetate to form the palladium complex [369,370]. The silver(I) and gold(I) complexes are accessible from the reaction with silver(I) oxide [371] and the usual carbene transfer reaction to gold(I) [372]. 

The high reactivity of this system is demonstrated by the fact that the cyclic ring system is cleaved by sodium hydroxide to give ( r-l,2-diphenylvinyl)phosphinic acid and is thermally unstable giving tolan at 120°C (fS9). The extension of this synthetic route to other phosphorus or other heteroatom derivatives has not yet been tested. 

As shown in Scheme 38, several primary alkyl-substituted cyclohexanones have been prepared by Lewis acid catalyzed phenylthioalkylation of the TMS enol ether of cyclohexanone followed by reductive removal of a phenylsulfenyl group. The two-step neopentylation sequence is particularly noteworthy. This methodology has been used to prepare numerous a-alkylated cyclic and acyclic ketones. a-Alkylated aldehydes can be produced in a like manner. a-Alkylidenation can also be accomplished by oxidative removal of sulfur. Lee and coworkers have found that TMS triflate-catalyzed reactions of silyl enol ethers of cyclic ketones and aldehydes with saturated and unsaturated l,l-dimethoxy-(i>-tri-methylstannanes, followed by addition of titanium tetrachloride, provide novel routes to fused and spiro-cyclic ring systems. Phenylthiomethylstannylations of silyl enol ethers have also been reported. ... 

In this example, it would be more convenient to maintain the substructure in the substructure file, and to generate smaller bi- or multi-cyclic ring systems by using portions of the larger molecule. 

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